Comment by WalterBright

2 days ago

As a former engineer who worked on the 757 flight control system, I am not terribly impressed with that design.

Having 3 pitot tubes iced over means they read 0 velocity. It is reasonable for the computer to be designed to recognize that if all three pitot tubes read 0, then the pitot tubes are the problem. With the altimeter unwinding, it should be able to recognize a stall. With the turn and bank indicator, and the AOA indicator, it should be able to return to straight and level.

Recall that the captain figured it out at a glance and knew exactly what to do.

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WalterBright

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manarth  2 days ago

The FAA report[1] gives a more comprehensive description of events.

The pitot tubes had differential icing, and didn't all read 0kts – they reported different velocity against each tube, such as 40kts or 60kts (against an expected baseline of ~ 275kts). The computer correctly recognised the data was invalid and rejected it.

It's a common narrative that the captain immediately figured out the issue. The report and transcript of the cockpit recording[2] notes that the captain's interventions showed that he had not identified the stall, nor had the copilots.

         ~ cockpit recording ~
    0:00 autopilot disconnects
    0:01 [copilot right] "I have the controls"
    0:11 [copilot right] "We haven't got a good display of speeds"
    1:26 captain enters cockpit
    1:30 [copilot right] "I don’t have control of the airplane at all"
    1:38 [captain] "Er what are you doing?"
    3:37 [captain] "No no don't climb"
    4:00 [captain] "Watch out you’re pitching up there"
    4:02 [copilot right] "Well we need to we are at four thousand feet"
    4:23 ~ recording stops ~



    [1] https://www.faa.gov/sites/faa.gov/files/AirFrance447_BEA.pdf
    [2] https://bea.aero/uploads/tx_elyextendttnews/annexe.01.en.pdf

  • WalterBright  1 day ago

    > he had not identified the stall

    Thank you. I had not seen the transcript before.

    Is it possible that 40/60 kts indicates a stall? Nevertheless, the drop in altitude while the nose was up should also indicate a stall.

    I know that designing avionics, and accounting for all possible scenarios is a difficult job, and we learn from the failures. But I don't buy that it was impossible/impractical for the avionics to figure out what was going on based what the other instruments were saying.

    • WalterBright  1 day ago

      Consider the inputs the avionics had:

      1. 40/60 kts on pitot tubes - could that be showing a flat spin?

      2. rotating compass - flat spin?

      3. altitude drop - stall

      4. attitude - level, no bank

      5. engines - operating normally

      6. GPS - no forward progress

      Does that add up to a stall?

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